The present invention relates generally to a printer having a thermal print head, and more particularly to a thermally printing apparatus wherein heat generating elements of a thermal print head are selectively activated to apply heat to segments of a thermally transferable ink layer on an ink ribbon and thereby effect a printing in a dot-matrix fashion on sheets of paper of an ordinary kind.
A printer using a thermally transferable ink is known according to Japanese patent application 54-139559 laid open for public inspection on May 27, 1981 (publication No. 56-62183), wherein there are provided two carriages which are disposed on opposite sides of a planar resilient member. The first carriage carries a thermal print head facing the resilient member, a positively driven presser roller, an idler roller, and a ribbon cassette accommodating a supply spool and a take-up spool. The first carriage is pivotable toward and away from the resilient member, i.e., between its printing and release positions, respectively. In the printing position, the two presser rollers and the thermal print head therebetween are held in pressed contact with one surface of the resilient member. An ink ribbon from the supply spool via the idler presser roller is fed in pressed contact with a sheet of paper between the thermal print head and the resilient member. A length of the ink ribbon past the thermal print head and the positively driven presser roller is re-wound on the take-up spool. The resilient member is sandwiched by the first carriage described above and the second carriage which backs the resilient member with its two support rollers of small diameter and another support roller of large diameter which are all held in pressed contact with the other surface of the resilient member. The first and second carriages are moved together through a common drive mechanism. The positively drive presser roller on the first carriage is driven by a pinion which is rotatably supported on the first carriage. The pinion is kept in engagement with a stationary rack parallel to a line of printing even while the first carriage is placed in the release position. Rotary movements of the pinion during movements of the first carriage (and the second carriage) in a printing direction are transmitted to the positively driven presser roller and the take-up spool through a one-way clutch which disconnects the power transmission while the pinion rotation is in the reverse direction during a return movement of the carriage. The positively driven presser roller and the pinion have the same outside diameter and are thus rotated at the same peripheral speeds. A torque limiter is incorporated in the transmission train linking the pinion and the take-up spool so that a torque exceeding an upper limit is not transmitted to the take-up spool. Another clutch is provided in the transmission train so that the movements of the pinion during the movements of the carriages in the printing direction are not transmitted to the positively driven pressure roller and the take-up spool when the first carriage is pivoted to its release position. Thus, the pinion in the above described printer known in the art is used to drive one of the presser rollers as well as the take-up spool, and the pinion is always engaged with the rack. The permanent rack and pinion engagement necessitates the use of a one-way clutch to stop feeding of the ink ribbon during the carriage return movement, and another clutch to stop the ribbon feed when the first carriage is pivoted to its release position (with the thermal print head away from the printing surface) even while the carriage is moved in the printing direction. The above discussed arrangements of the double-carriage design, including disconnectable transmission lines between the pinion and the presser roller and take-up spool, and use of several rollers pressed against the resilient member, will inherently complicate the structure of the printer, and increase the number of component parts and consequently push up the cost of manufacture thereof.
It is also recognized, in the art of printing using an ink ribbon coated with a thermally transferable ink layer, that a slack of the ink ribbon on the side of the supply spool owing to rotary movements of the supply spool due to inertia or vibration will cause the ink ribbon to take a waving or meandering path or have creases during a continuous printing and results in a trouble of jamming of the ribbon adjacent a printing station. Such jamming trouble is serious especially when a long underline is drawn or a succession of letters "T" is printed. In this condition, a lower or upper portion of the ribbon is subject to more heat from the print head, and consequently has more fusion of the ink layer, which causes different degrees of adhesive forces due to the fused ink materials at the upper or lower portion of the ribbon transversely of the ribbon width, thereby subjecting one of the upper and lower width portions to more tension than the other. Thus, the ribbon tends to have creaes or be offset from its normal path, i.e., brought out of alignment with the heat generating elements of the print head, causing an unclear printing of characters or printing failure at the top or bottom of the characters.
To overcome the above problem, it has been proposed to provide the supply spool with biasing means for applying tension to a portion of the ribbon adjacent the supply spool in a direction opposite to a direction of the ribbon feed in order to prevent the ribbon from being slackened. This solution to the prior problem, however, has a disadvantageous aspect that the tension applied to the ribbon acts to prevent the ribbon from faithfully following a speed of a relatively movement between the ribbon cassette and the paper, and therefore the ribbon tends to slip in frictional contact with the printing surface of the paper, which results in staining or soiling the printing surface with the ink. Further, the tension applied by the biasing means counteracts a winding force of the take-up spool and thus prevents a correct or sufficient winding action of the take-up spool especially when the printer is powered by a battery and the take-up spool is driven with a small torque.